This project focuses on the transcription factors HB9 and BSAP, originally identified in B lymphocytes. The HB9 gene also referred to as HlxB9 encodes a homeodomain coding protein. Gene targeting in mice has revealed a critical role for HB9 in motor neuron and pancreas development. Abnormalities in the sacral region occurs in humans with one abnormal HB9 allele. Mice heterozygotic for an HB9 null mutation are normal, but mice that lack HB9 die at birth of respiratory failure due to a lack of diaphragm innervation. The motor neuron defects may arise from the inappropriate expression of genes normally expressed in other types of neurons. These mice also lack a portion of their pancreas, the dorsal lobe, and have reduced numbers of insulin producing beta cells in their residual ventral lobe. Because the developmental defects and perinatal lethality makes assesmment of the function of HB9 in adult tissues impossible, we have used a conditional gene targeting approach and have introduced loxP sites on either side of exon 3 of the HB9 gene. Such mice are being tested to assure that the loxP sites do not interfere with normal HB9 expression. If not, these mice can be used to conditionally inactive HB9 by breeding them to Cre transgenic mice, which express Cre in a temporal and/or tissue specific fashion. In addition, to assist in the analysis of HB9 expression in developing and adult tissues the gene which encodes the green fluorescent protein (GFP) has been introduced into the HB9 locus. Also, the HB9 mutation has been back crossed on to a C57Black/6 background. By reconstituting lethally irradiated Rag -/- C57Black/6 mice with with fetal liver cells prepared from HB9 -/- deficient embryos we can assess whether HB9 is important for normal lymphopoiesis and lymphocyte function. Finally to identify HB9 target genes a genomic array approach has been used. Chips containing oligonucleotides from 15,000 different genes have been analyzed using mRNA prepared from wild type and HB9 deficient embryos. Alterations in gene expression will be verified by in situ hybridization and immunohisto-chemistry. BSAP is a transcription factor critical for B-lymphocyte development and lineage committment. Mice overexpressing BSAP have B cells which are hyperproliferative, have an apoptosis defect, and a propensity for developing lymphomas. Over the past year we have documented the frequency of lymphomas in these mice. We plan to use genomic arrays to analyze gene expression in the BSAP transgenic B cells.